This invention relates generally to signal amplification and, in particular, to determining signal amplifier characteristics for intentionally induced distortion techniques utilized prior to and in conjunction with signal amplification.
In the field of radio communication systems, it is a well-known problem that the power amplifiers present in transmission equipment operate in a non-linear fashion when the power amplifiers are operated near their peak output. As a result, the power amplifier introduces significant signal distortion that can appear in various forms. For example, if more than one signal is input into the power amplifier or power amplification stage, its non-linear characteristics can cause an undesirable multiplicative interaction of the signals being amplified, and the power amplifier""s output can contain intermodulation products. These intermodulation products cause interference and crosstalk over the power amplifier""s operational frequency range.
In power amplifier design, there is a trade off between distortion performance and efficiency. Linear amplifiers that operate under xe2x80x9cClass Axe2x80x9d conditions create little distortion but are inefficient, whereas non-linear amplifiers operated under xe2x80x9cClass Cxe2x80x9d conditions are reasonably efficient but introduce significant distortions. While both efficiency and distortion are important considerations in amplifier design, efficiency becomes increasingly important at high power levels. Because of their efficiency, non-linear amplifiers are largely preferred, leaving the user with the problem of distortion.
In order to employ non-linear power amplifiers, techniques have been used to improve linearity and thereby reduce the effects of interference and crosstalk. Linearity can be achieved by application of various linearization techniques that reduce the distortion caused by non-linear amplification. Conventional amplifier linearization techniques can be broadly categorized as feedback, feedforward, or predistortion.
The last mentioned technique, predistortion, intentionally distorts the signal before the power amplifier so that the non-linearity of the power amplifier can be compensated. According to this technique, linearization is achieved by distorting an input signal according to a predistortion function in a manner that is inverse to the amplifier characteristic function. The predistortion technique can be applied at radio frequency (RF), intermediate frequency (IF), or at baseband.
In the baseband domain, the input signal information is at a much lower frequency, allowing digital methods to be employed. The predistortion function is applied to the input signal with the resulting predistorted signal being upconverted to IF and then finally to the RF carrier frequency. It is also possible to apply adaptive predistortion techniques where feedback from the output of the amplifier is used to update and correct the predistortion function.
The form of the predistortion function is dependent upon the model used to characterize the output of the amplifier. Predistortion functions in the baseband domain are typically implemented as a table of gain and phase weighting values within a digital signal processor. A Cartesian feedback method employs a quadrature representation of the signal being amplified. The incoming quadrature signals I and Q are compared to the feedback quadrature signals. Thus, there are two sets of coefficients, one for each quadrature channel, that are being updated to model the predistortion characteristics. In this manner, gain and phase non-linearities within the amplifier can be compensated. Performance is dependent on the size of the look up table and the number of bits used to represent the signal. Better performance and more adaptivity is achieved with larger look up tables and more bits albeit at the expense of longer processing times.
Predistortion functions are also modeled as polynomials. Ideal amplifiers have linear characteristics; consequently, amplifiers with slight non-linearities can be modeled as polynomials of only a few terms, with only odd terms being employed. Even terms are discarded because their use with negative-valued inputs can interfere with linearity. While processing demands are eased by excluding and limiting the number of terms in the polynomial modeling, performance is sacrificed.
Accordingly, there is a need for a device to more quickly and efficiently determine the characteristics of a non-linear amplifier.
The present invention teaches an apparatus and method for modeling and estimating the characteristics of a power amplifier. A predistortion module generates a predistorted signal in response to a predistortion function and an input signal. A power amplifier receives the predistorted signal and generates an output signal. A polynomial module generates coefficients of a complex polynomial of order p (p is an integer greater than one) in response to the predistorted signal and the output signal. The coefficients characterize the power amplifier. The complex polynomial is implemented with both even and odd terms. Even order terms are typically ignored for the polynomial modeling of amplifiers because the non-linear distortion signals caused by power amplifiers are strongest at odd order harmonic frequencies. Additionally, it is commonly thought that the use of even terms with negative valued inputs can interfere with the linearity, since negativity is lost at even powers. However, the inclusion of even order terms improves the ability to accurately model the power amplifier and thereby improves predistortion performance.
In another exemplary embodiment of the present device, the polynomial module employs a minimum mean squared error criteria to determine said polynomial coefficients, thereby allowing a very fast and efficient implementation.
By improving the ability to model power amplifiers, the present invention improves the ability to model the power amplifier predistortion function. The invention further enables power amplifiers to be operated in the non-linear region near saturation, yet suppresses undesirable intermodulation products. Resort to a larger amplifier, to keep operation within the linear region, is avoided. Power amplifier sizes are kept small with associated cost savings, particularly important in the field of wireless communications.
The above factors make the present invention essential for effective power amplifier predistortion.